1. Field of the Invention
The present invention is directed in general to the field of semiconductor devices. In one aspect, the present invention relates to the fabrication of high-k metal gate semiconductor devices.
2. Description of the Related Art
As the size and scaling of semiconductor device technology is reduced, aspects of device design and fabrication that previously gave rise to only second-order effects in long-channel devices can no longer be ignored. For example, the reduced scaling of channel length and gate oxide thickness in a conventional MOS transistor exacerbates problems of polysilicon gate depletion, high gate resistance, high gate tunneling leakage current and dopant (i.e., boron) penetration into the channel region of the device. As a result, CMOS technology is increasingly replacing silicon dioxide gate dielectrics and polysilicon gate conductors with high dielectric constant (high-k) dielectrics in combination with metal gate electrodes formed from a gate stack of polysilicon and one or more metal layers. With such technologies, the metal gate layers not only obviate gate-depletion and boron-penetration effects, but also provide a significantly lower sheet resistance.
While high-k dielectrics in conjunction with metal gate electrodes advantageously exhibit improved transistor performance, the use of new gate electrode technologies can create new technical challenges. For example, when high-k capping dielectric layers (e.g., La2O3) are to be selectively formed over only portions of the device substrate, conventional techniques have called for the application and patterning of photoresist materials directly on top of the high-k capping dielectric layer in areas to be protected against wet etching, but this often results in poor adhesion and cracking of the photoresist layer. Attempts have been made to promote photoresist adhesion, but such techniques are ill-suited for use with high-k gate dielectric materials (such as HfO2) or have otherwise not proven workable. For example, ozone and O2-based plasmas have been used to help oxidize surfaces and promote adhesion with photoresist, but such oxygen-based plasmas cause interface layer re-growth which increases the gate oxide inversion thickness (Tiny) and can not be used to promote photoresist adhesion at this level. In addition, developable bottom antireflective coating (dBARC) photoresist layers have been proposed for application directly to the dielectric capping layers to avoid photoresist adhesion problems, but it is difficult to control the photoresist profiles with currently-available dBARCs which typically result in more defects than conventional photoresists. Other approaches of improving adhesion—including using an adhesion promoter (e.g., hexamethyldisilazane), post-develop bake recipes, and other modifications to the litho track recipes—have only shown minor improvements.
Accordingly, a need exists for an improved metal gate electrode and manufacture method for incorporated very thin high-k gate dielectric materials in NMOS and PMOS that addresses various problems in the art that have been discovered by the above-named inventors where various limitations and disadvantages of conventional solutions and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow, though it should be understood that this description of the related art section is not intended to serve as an admission that the described subject matter is prior art.